On Fri, Jan 27, 2023 at 3:17 PM Andres Freund <and...@anarazel.de> wrote: > > Hi, > > On 2023-01-27 14:24:51 +0900, Masahiko Sawada wrote: > > If I'm understanding this result correctly, it seems to me that your > > patch works well with the WAL DIO patch (WALDIO vs. WAL DIO & WAL > > BUFFERS READ), but there seems no visible performance gain with only > > your patch (HEAD vs. WAL BUFFERS READ). So it seems to me that your > > patch should be included in the WAL DIO patch rather than applying it > > alone. Am I missing something? > > We already support using DIO for WAL - it's just restricted in a way that > makes it practically not usable. And the reason for that is precisely that > walsenders need to read the WAL. See get_sync_bit(): > > /* > * Optimize writes by bypassing kernel cache with O_DIRECT when using > * O_SYNC and O_DSYNC. But only if archiving and streaming are > disabled, > * otherwise the archive command or walsender process will read the > WAL > * soon after writing it, which is guaranteed to cause a physical > read if > * we bypassed the kernel cache. We also skip the > * posix_fadvise(POSIX_FADV_DONTNEED) call in XLogFileClose() for the > same > * reason. > * > * Never use O_DIRECT in walreceiver process for similar reasons; the > WAL > * written by walreceiver is normally read by the startup process soon > * after it's written. Also, walreceiver performs unaligned writes, > which > * don't work with O_DIRECT, so it is required for correctness too. > */ > if (!XLogIsNeeded() && !AmWalReceiverProcess()) > o_direct_flag = PG_O_DIRECT; > > > Even if that weren't the case, splitting up bigger commits in incrementally > committable chunks is a good idea.
Agreed. I was wondering about the fact that the test result doesn't show things to satisfy the first motivation of this patch, which is to improve performance by reducing disk I/O and system calls regardless of the DIO patch. But it makes sense to me that this patch is a part of the DIO patch series. I'd like to confirm whether there is any performance regression caused by this patch in some cases, especially when not using DIO. Regards, -- Masahiko Sawada Amazon Web Services: https://aws.amazon.com
